The present invention relates to an input/output control system for data transmission and reception between a central processing unit and a plurality of input/output units via a common bus under the control of the central processing unit.
In this kind of input/output (hereinafter referred to as I/O) control system heretofore employed, a central processing unit (hereinafter referred to as CPU) is connected to buses, to which I/O units are connected via individual I/O interfaces. As the buses use is made of a data bus for transmitting data, an address bus for transmitting an address signal for selecting a desired one of the I/O units and a control bus for transmitting control signals such as a read/write control signal, an interrupt request acknowledgment signal, an interruption inhibit signal, a reset signal, a trigger signal and an interrupt request signal. In addition, since each bus transmits the signal in the form of parallel signals, the number of core conductors used for each bus is appreciably large, resulting in a bus cable becoming thick. Furthermore, in order to increase the number of bits of one data signal or to increase the number of bits of one address signal with the view of increasing the number of selectable I/O units, the number of core conductors of the data bus or address bus must be increased and, in this case, it is usually necessary to replace the bus with a new one. In practice, however, such replacement of the bus is very difficult. Moreover, in the prior art system, since data is transmitted over a common data bus in both directions, there is the possibility that if mismatching exists in the joint between the data bus and each I/O unit, a reflected wave is generated to cause erroneous data transmission reception. Such an error increases with an increase in the number of I/O interfaces connected to the data bus, and the influence of this error becomes marked in the case of transmitting and receiving signals at high speed. Besides, since signals such as the data signal and so forth are transmitted over the common data bus in two directions as mentioned above, it is necessary to provide two-way drivers for the bus; this leads to an increase in the cost of the entire system. Also, such a driver must be provided for each core conductor of each bus, and consequently, the number of such drivers used is very large. Further, since the signal sending-out rate of the CPU is determined on the basis of the time necessary for a signal from the CPU to reach an I/O unit located at a place remotest from the CPU, the signal sending-out rate must be reduced as the transmission distance increases. This imposes a limitation on the data transmission rate, making it impossible to achieve high-speed data transmission.
It is an object of the present invention to provide an I/O control system which requires less core conductors in the bus connecting the system.
Another object of the present invention is to provide an I/O control system in which the number of bits of a data signal, i.e. the number of bits of one word, or the number of bits of an address signal, can be increased, and consequently the number of I/O units can be increased without increasing the number of core conductors forming the bus.
Another object of the present invention is to provide an I/O control system which permits data transmission free from the influence of a reflected wave occurring on the bus, that is, without disturbing the waveform of a transmitted signal by the reflected wave, thus enabling an increase in the transmission distance.
Another object of the present invention is to provide an I/O control system which permits high-speed data transfer regardless of the distance between a CPU and an I/O unit which is the remotest therefrom.
Yet another object of the present invention is to provide an I/O control system which requires less core conductors forming each of signal buses and consequently requires less drivers for driving the signal bus and, in addition, employs only one-way drivers as such drivers, and hence is simple-structured and inexpensive.